1. Field of the Invention
The present invention relates to multi-channel access type radio telephone devices which include a base unit connected to a cable telephone line and a radio telephone connected via a radio circuit line with the base unit, wherein an idle one of a plurality of radio channels provided between the base unit and the radio telephone is detected to transmit/receive voice and control signals, etc., and methods of controlling the radio telephone devices, and more particularly to such devices which reduce the level of the control signal leaking possibly to the cable telephone line, especially in the speech holding and terminating operation, etc.
2. Description of the Related Art
FIG. 1 is a block diagram of the structure of a conventional radio telephone device of this type. In FIG. 1, a base unit 1 is connected via a radio circuit line with a radio telephone 2 and also with a cable telephone line 3.
The incoming signal from the cable telephone line 3 is delivered as a modulating input to a transmitter 7 via a line switch 4, a hybrid circuit 5 and a transmission voice switch 6. The electric waves modulated by the transmitter 7 are radiated toward the radio telephone 2 from a transmission antenna 8. The transmitted electric waves from the radio telephone 2 are received by a reception antenna 9 and demodulated by a receiver 10. The demodulated signal is delivered to the cable telephone line 3 via a voice switch 11, the hybrid circuit 5 and the line switch 4.
A synthesizer 12 outputs a signal of a frequency, corresponding to a radio channel (a speech channel and a control channel), to the transmitter 7 and receiver 10.
One of the demodulated outputs from the receiver 10 is input to a reception electric field detector 13 and used for detection of the field strength. The detector circuit 13 may be constituted by a carrier squelch circuit or a noise squelch circuit. Another demodulated output from the receiver 10 is output as a data signal contained in the received demodulated waves and input to an identification signal detector 14 which checks an identification signal (ID code) determined by a combination of the base unit 1 and radio telephone 2.
The output from the field detector 13 and identification signal detector 14 and the demodulated data signal output are input to a control circuit 16 for use in connection control. The control circuit 16 controls the synthesizer 12 to control the switching of radio channels and sends the transmission data signal as an modulating input to the transmitter 7, etc. The holding circuit 15 sends a melody sound to the cable telephone line 3 when the holding operation (by a holding button 29a of a key pad unit 29) is performed at the radio telephone 2.
Similarly, the radio telephone 2 includes a reception antenna 18 and a receiver 19 the demodulated output of which is delivered to an earpiece 20. Voice to a mouthpiece 21 acts as a modulating input to the transmitter 22 the output of which is radiated from a transmission antenna 23.
A synthesizer 34, a reception electric field detector 24 and an identification signal detector 25 are similar to those in the base unit 1. A control circuit 27 controls the radio telephone 2. A hook-switch 28 is operated when a call is placed, when a speech is terminated and when the output signal from the switch 28 is input to the control circuit 27. A dial key pad unit 29 is for a dialing operation and line holding purposes and the output signal from the key pad unit is input to the control circuit 27. A speaker 30 is a sounder which generates a ringing tone when there is an incoming call. In the base unit, a power supply for these elements is obtained from a rectifying and stabilizing circuit 32 via an AC plug 31 connected to an AC 100 volt source. The radio telephone 2 uses as a power supply the output of a rechargeable battery 33.
The control of this conventional radio telephone device is provided as follows when there is an incoming call.
When the base unit 1 in a standby state detects a call signal from the cable telephone line 3, it sets the oscillating frequency of the synthesizer 12 at a control channel frequency, and turns on the transmitter 7 to cause same to transmit an incoming signal. The incoming signal includes a signal to designate a speech channel S-CH.
The radio telephone 2 turns on the synthesizer 34 for a predetermined time when it is in a standby state, sets its oscillating frequency at the control channel frequency and turns on the receiver 19. When the incoming signal is received at this time, the radio telephone 2 turns on the transmitter 22 to cause same to transmit an acknowledgement signal including an ID code allocated to the telephone 2 and selects a speech channel S-CH designated by the base unit 1.
When the base unit 1 detects the electric waves from the radio telephone 2 using the reception field detector 13, it stops the transmission of the incoming signal. When it confirms the ID code contained in the acknowledge signal, it selects a channel S-CH designated by the incoming signal. If not, the base unit 1 awaits until the calling from the cable telephone line 3 disappears because there may be a response from another radio telephone set, and then returns to its standby state.
After the base unit 1 selects the channel S-CH, it sends a bell ringing signal. When the radio telephone 2 receives it, it sends a ringing tone from the sounder 30. When the hook switch 28 goes off-hook by its on operation in response to the ringing tone, the radio telephone 2 sends an off-hook signal to thereby establish speech communication.
When the base unit 1 receives the off-hook signal, it stops the transmission of the bell ringing signal and closes the line switch 4 to establish a speech loop with the cable telephone line 3 for speech purposes.
The operation of the radio telephone set 2 performed when the telephone 2 places a call is as follows. When the hook-switch 28 is turned on for placing a call to the cable telephone line 3, the control circuit 27 determines that it should shift to the calling operation because the switch 28 is turned on, locks the oscillating frequency of the synthesizer 34 at the control channel, turns on the receiver 19 to cause same to receive electric waves at the control channel C-CH. The reception field detector 24 detects the field strength of the received electric waves in the control channel. If the detected field strength does not exceed a predetermined reception field strength, the radio telephone 2 determines that the control channel is idle and turns on the transmitter 22 to cause same to transmit a call signal including the ID code allocated to the telephone 2. The base unit 1 monitors the presence of the received electric waves in the control channel. When the base unit 1 receives the call signal from the radio telephone 2, it determines whether the ID code contained in the call signal coincides with the ID code allocated to the combination of the radio telephone 2 and the base unit. If so, it turns on the transmitter 7 to cause same to transmit a call responsive signal (including the ID code and information on the designation of a S-CH) to the radio telephone 2.
The radio telephone 2 receives the call responsive signal from the base unit 1 via the control channel and checks whether the ID code contained in the call responsive signal coincides with that of the radio telephone 2. If so, it switches the oscillating frequency of the synthesizer 34 to that of the speech channel designated by the base unit 1. After the base unit 1 has transmitted the call responsive signal, it switches the oscillating frequency of the synthesizer 12 to the frequency of the speech channel designated for speech purposes.
Thus, the base unit 1 and radio telephone 2 are connected by the speech channel designated by the base unit 1. By a subsequent dialing operation through the dial key pad unit 29, the telephone connected to the cable telephone line 3 is called and placed into telephonic communication.
If the telephonic communication ends and the hook switch 28 goes on-hook thereafter, the control circuit 27 inputs a speech terminating signal as a shut off command to the transmitter 22 to cause same to radiate the terminating signal from the transmission antenna 23 in order to interrupt the connection of the radio telephone 2 with the cable telephone line 3.
When the control unit 16 of the base unit 1 receives the terminating signal via the receiver 10, it first turns off the voice switch 11 to interrupt the voice path between the receiver 10 and the hybrid circuit 5. If the ID code contained in the terminating signal coincides with a predetermined allocated one, and the control command is a "speech terminating" command, the control circuit 16 turns off the line switch 4 to interrupt the connection with the cable telephone line 3.
The terminating signal transmitted by the radio telephone 2 is a data signal of a predetermined bit length and constituted by a bit synchronizing signal 41, a frame synchronizing signal 42, an ID code signal 43 and a speech terminating command signal 44, as shown in FIG. 2(a). When the bit synchronizing signal 41 and the frame synchronizing signal 42 are successively detected, the control circuit 16 of the base unit 1 turns off the voice switch 11 as shown in FIG. 2(b). Subsequently, when the ID code signal 43 coincides with that allocated to the base unit and the control command is the speech terminating command signal 44, the base unit turns off the line switch 4 as shown in FIG. 2(c). Thereafter, the base unit 1 transmits a speech termination responsive signal to the radio telephone 2 to thereby return the radio telephone 2 to its standby state.
As will be obvious from the timing chart of FIG. 2, the control circuit 16 of the base unit 1 turns off the voice switch 11 when the frame synchronizing signal 42 is detected in the speech termination. Therefore, the bit and frame synchronizing signals 41 and 42 are delivered to the cable telephone line 3 as shown in FIG. 2(d) during the time interval t from the transmission of the bit synchronizing signal 41 to the turning off the switch 11. Thus, the control signal unrelated to the communication voice would leak to the cable telephone line 3, which does not become a big problem if the signal level of the bit synchronizing signal 41, etc., is low. However, since the level of control signals such as the speech terminating signal is rendered high to ensure a sufficient S/N ratio and hence sufficient reliability even if the radio telephone 2 is positioned close to the bound of the service area in the past, the level of the control signal leaking possibly to the cable telephone line 3 is considerably high. Therefore, this would act as an uncomfortable sound disadvantageously to the caller on the cable telephone line.
A similar phenomenon would also occur during the time interval from the turning off of the voice switch 11 by the holding operation to the generation of a melody sound indicative of the hold state to thereby give an uncomfortable feeling to the addressee.
Since the addressee continues to hold the handset in telephonic communication during the holding operation, the uncomfortable sound greatly influences the addressee.